SQL Server: EXISTS Condition

In this post explains how to use the EXISTS condition in SQL Server (Transact-SQL) with syntax and examples.

The SQL Server (Transact-SQL) EXISTS condition is used in combination with a subquery and is considered to be met if the subquery returns at least one row. It can be used in a SELECT, INSERT, UPDATE, or DELETE statement.

Syntax

The syntax for the EXISTS condition in SQL Server (Transact-SQL) is:

WHERE EXISTS ( subquery );

Parameters or Arguments

subquery

The subquery is a SELECT statement. If the subquery returns at least one record in its result set, the EXISTS clause will evaluate to true and the EXISTS condition will be met. If the subquery does not return any records, the EXISTS clause will evaluate to false and the EXISTS condition will not be met.

Note

• SQL statements that use the EXISTS condition are very inefficient since the sub-query is RE-RUN for EVERY row in the outer query's table. There are more efficient ways to write most queries, that do not use the EXISTS condition.

Example - With SELECT Statement

Let's look at a simple example.

The following is a SELECT statement that uses the EXISTS condition:

SELECT *
FROM employees
WHERE EXISTS (SELECT *
              FROM contacts
              WHERE employees.last_name = contacts.last_name
              AND employees.first_name = contacts.first_name);

This SQL Server EXISTS condition example will return all records from the employees table where there is at least one record in the contacts table with a matching last_name and first_name.

Example - With SELECT Statement using NOT EXISTS

The SQL Server EXISTS condition can also be combined with the NOT operator.

For example,

SELECT *
FROM employees
WHERE NOT EXISTS (SELECT *
                  FROM contacts
                  WHERE employees.last_name = contacts.last_name
                  AND employees.first_name = contacts.first_name);

This SQL Server EXISTS example will return all records from the employees table where there are no records in the contacts table for the matching last_name and first_name.

Example - With INSERT Statement

The following is an example of an INSERT statement that uses the EXISTS condition:

INSERT INTO contacts
(contact_id, contact_name)
SELECT supplier_id, supplier_name
FROM suppliers
WHERE EXISTS (SELECT *
              FROM orders
              WHERE suppliers.supplier_id = orders.supplier_id);

Example - With UPDATE Statement

The following is an example of an UPDATE statement that uses the EXISTS condition:

UPDATE suppliers
SET supplier_name = (SELECT customers.name
                     FROM customers
                     WHERE customers.customer_id = suppliers.supplier_id)
WHERE EXISTS (SELECT customers.name
              FROM customers
              WHERE customers.customer_id = suppliers.supplier_id);

Example - With DELETE Statement

The following is an example of a DELETE statement that uses the EXISTS condition:

DELETE FROM contacts
WHERE EXISTS (SELECT *
              FROM employees
              WHERE employees.last_name = contacts.last_name);